A flat panel display possesses advantages of being ultra thin, power saved and radiation free. It has been widely utilized. A present flat panel display mainly comprises a LCD (Liquid Crystal Display) or an OLED (Organic Light Emitting Display).
An OLED possesses many outstanding properties of self-illumination, no requirement of backlight, high contrast, ultra-thin, wide view angle, fast response, applicability of flexible panel, wide range of working temperature, simpler structure and process. Therefore, the OLED is considered as next generation flat panel display technology. As considering the molecular weight of the organic electroluminescence material, the organic electroluminescence elements can be categorized as Organic Light Emitting Diode (OLED) and Polymer Light Emitting Diode (PLED). The molecular weights are different and the manufacture processes of the organic electroluminescence elements are significantly different. OLED is mainly manufactured by thermal deposition. PLED is manufactured by thermal deposition or inkjet printing.
Generally, OLED comprises a substrate, an ITO transparent anode located on the substrate, a Hole Injection Layer (HIL) located on the ITO transparent anode, a Hole Transporting Layer (HTL) located on the Hole Injection Layer, an Emitting Material Layer (EML) located on the Hole Transporting Layer, an Electron Transport Layer (ETL) located on the Emitting Material layer, an Electron Injection Layer (EIL) located on the Electron Transport Layer and a cathode located on the Electron Injection Layer. For promoting the efficiency, the emitting material layer is generally applied with co-host system.
Semiconductor nanocrystals (NCs) mean the semiconductor nanocrystal particles with size of 1-100 nm. Because the size of the semiconductor nanocrystals is smaller than the Exciton Bohr Radius of the material. Strong quantum confinement effect appears. The quasicontinuum evolves to become similar to the discrete energy level of the molecules and shows new material properties. Therefore, it is so called quantum dots (QDs). With the excitation of the external energy (photoluminescence, electroluminescence, cathode ray luminescence and etc.), the electrons jumps from the ground state to the excited state. The electrons and the electron holes in the excited state may form excitons. The electrons and the electron holes generate recombinations and ultimately relax to the ground state. The supernumerary energy may irradiate and generate photons with the processes of the recombination and relaxation.
The Quantum Dots Light Emitting Diodes (QD-LEDs) have significant commercial application values and cause strong research interests of the people in the recent decay. In fact, QD-LEDs possess many advantages in comparison with Organic Light Emitting Diodes (OLEDs): (1) The luminous line width of the quantum dots is between 20-30 nm. The FWHM is narrower compared with the luminescence of the organic electroluminescence >50 nm which functions as the key to achieve the great color purity of the image. (2) The inorganic material shows a better heat stability than that of the organic material. The Joule heat is the main reason to make the elements degenerated when the elements are under high brightness or high current density. With the excellent heat stability, the elements of inorganic material show longer usage time. (3) The color of the OLED display changes with time because the life times of the organic materials of three primary colors, red, green and blue. However, quantum dots are synthesized to have different size by one kind of material to realize the light emitting of the three primary colors. Similar degeneration life time can be obtained by using the same kind of material. (4) QD-LEDs is capable of realizing the light emitting of the red light and the wavelength of the organic material is generally smaller than 1 micrometer. (5) For the quantum dots, there is not restriction for the spin-statistics and the external quantum efficiency (EQE) is possible to reach up to 100%. The EQE of the QD-LEDs can be indicated as: ηExt=ηr*ηINT*η*ηOUT. The ηr is the probability that the electrons and the electron holes form the excitons. The ηINT is the internal quantum efficiency, i.e. the photoluminescence quantum yield (PL QY). The η is the probability of the radiative jump. The ηOUT is the efficiency of the external coupling. The restriction of the organic fluorescent dye ηr is 25%. The formation ratio of the single state and the triplet state is 1:3 and only the recombination of the single state excitons results in luminescence. Then, with the spin orbit coupling, the ηr of the organic phosphorescent material can be larger than 25%. Significantly, the organic phosphorescent material causes the degeneration of the fertile material. The ηOUT of the flat panel display is about 20% and the efficiency of the external coupling can be raised with the microcavity structure. For the QD-LEDs, the ηINT can reach up to 100%. Meanwhile, the ηr can reach up to 100% when the energy level of the electron and electron hole befit.
The Quantum Dots Light Emitting Diodes (QD-LEDs) can comprise organic inorganic hybrid elements and full inorganic elements. The former can realize high brightness and flexible applicability. The latter possess great advantage of element stability. There are two common development directions for the colorful OLED. One is RGB, three primary color luminescence. Samsung is the representative. The skill is merely applicable to the organic small molecular material which easily sublimates. The merit is that the art is simple, mature and easy for operation. Nevertheless, as manufacturing the high resolution display screens, high accuracy mask and precise alignment are required. Consequently, low productivity and high manufacture cost are the results. The other is white light+RGB filters skill. LG is the representative. The mature CF skill of LCD can be utilized without the mask alignment. The vapor deposition is tremendously simplified and the manufacture cost is possibly reduced which is applicable for manufacturing the large size, high resolution OLEDs. However, most of the luminous energy is absorbed by the filters and only 30% of the light can penetrate through. The high efficient white light material becomes essential. Otherwise, the element efficiency is lower and generally can be only applied for small molecule OLED display screens.